The present invention relates to the art of electric arc welding and particularly a starting device for use in a DC TIG welder operated by an SCR input and capable of sustaining a welding arc at a low current.
In TIG welding a tungsten electrode is spaced from a workpiece to define a gap across which an arc is created for melting a filler wire moved into the arc. This type of welding presents problems relating to the starting of the arc at the start of the TIG welding process. Several procedures are known to start the arc when the TIG welder is operated at over about 5 amperes. Starting procedures and background technology regarding arc starting is discussed in De Coster U.S. Pat. No. 6,075,224. This patent illustrates an arc starter circuit using high frequency. As background information, this patent is incorporated by reference. The present invention relates to a high current DC TIG welder which is operable at a low weld current, such as a current in the general range of 2-5 amperes, but with capability for lower minimum output. The architecture of the TIG welder employed in practicing the present invention involves a power supply with a background stage used in conjunction with a standard phase controlled constant current stage. The use of an auxiliary or background power supply is shown and discussed in De Coster. Such an architecture for a welder is also disclosed and described in Campiotti U.S. Pat. No. 4,950,864, Vogel U.S. Pat. No. 5,218,182, Terayama U.S. Pat. No. 5,645,741 and Heraly U.S. Pat. No. 6,034,350. These patents are incorporated by reference herein to describe the operation of a background, rectified power supply used in combination with a standard phase controlled power supply for TIG welding. The background power supply maintains a minimum current and the main power supply provides weld current above a minimum level for the actual welding process. The present invention utilizes a background current stage and details of such operation are not necessary in view of the well known technology set forth in these several patents.
DC TIG welders powered by a single phase SCR rectified welding current have an adjustable welding current determined by the phase firing angle of the SCRs in the network of the output rectifier. For industrial use they have adjustable welding DC currents as high as 200-300 amperes. As the desired welding current is reduced to a low level, such as 5-10 amperes, the phase angle is shifted to produce a reduced amount of the current flow. Such low current operation is not conducive to arc stability because the SCRs are on for a short time; therefore, a secondary power supply creating a fixed minimum current is often used in conjunction with the phase controlled rectifier. The low current operation includes a series of small current pulses in combination with a fixed background current. This architecture presents difficulty at low welding currents because the standard procedures for starting the arc is not consistent at low currents. Consequently, in the past the lowest current which could be used in a DC TIG welder of the type capable of operating at 200-300 amperes and driven by a single phase SCR platform was about 5-10 amperes. At this low weld current, high frequency or scratch arc starting, as discussed in De Coster U.S. Pat. No. 6,075,224, was marginally acceptable. If the main power supply stage was operated at a high level for a short time to start the TIG welder, the minimum current operation was still 5 amperes or more because of the short SCR spikes during the welding operation. There is no SCR driven TIG welder which can be successfully started at low currents below 5 amperes and then operated at such level with a stable arc. Such low welding current is the optimum current for certain welding processes, such as welding thin aluminum parts. Low current for DC TIG welding means a welding current less than about 5 amperes, and in the general range of 2-5 amperes. The power supply for a DC TIG welder can be adjusted to a low current by reducing the conduction period of the SCRs; however, there is no dependable procedure for starting the DC TIG welding process at such low currents and then maintaining the arc stability since the low current involves merely short spikes from the SCRs. This is especially true when the welder is designed to operate at current levels of 200-300 amperes when the SCRs are nearly fully conductive.
The invention involves a DC TIG welder driven by a single phase SCR platform and having a capacity of up to 100-300 amperes. The welding current of the welder can be set to a low level, substantially less than 5 amperes and in the general range of 2-5 amperes. At such low current, the welder now provides a welding current waveform similar to an inverter based welder. The invention employs a power supply having a phase control rectifier stage and a background rectifier stage. The background rectifier is set at the desired low current to be used for low current welding. The starting device of the welder employs both stages for starting, but maintains the arc using only the low current stage of the power supply. Consequently, the SCRs are not used during minimum current operation. A primary aspect of the invention is a high capacity welder that uses the phase control stage of the power supply only during starting when the welder is set for low current operation. While providing low current from the second rectifier a novel start sequence is implemented by a start device using any software or hardware technique. A high frequency generator passes a high frequency, high voltage signal across the gap between the workpiece and electrode. A start pulse with a controlled peak current in the general range of 50-70 amperes is created by short term operation of the SCR rectifier. By combining the standard high frequency arc starting procedure with the controlled arc starting pulse, the arc is positively created and stabilized. The pulse is tailored to optimize the creation of the arc by having the controller progress to a peak current of 50-70 amperes, then tail off. The duration is generally 30-50 milliseconds. This includes about 2-4 firings by the SCRs of the first rectifier. The high frequency complements the controlled short, high current start pulse to assure ionization of the gap before the pulse is applied across the gap by the first rectifier. After the current pulse has created the arc, high frequency is terminated. The high frequency is discontinued for welding at the fixed low current established by the background stage of the power supply using no SCR currents.
If the desired weld current is adjusted above the set low current of the second rectifier, the main SCR rectifier is initiated and the SCR network is used. The invention has proven satisfactory in TIG welding to a level as low as 1 ampere with a SCR based welder having a high current capability. The welder operated at 2 amperes maintains a well-defined arc created by the high voltage start pulse of the present invention without disrupting SCR spikes associated with low current welding by such high capacity DC welder. Although starting procedures known in the prior art are successful for high welding currents in an SCR based DC TIG welder, such starting procedures are not capable of establishing an arc for low current TIG welding and then sustaining the arc without use of SCR current spikes. The present invention accomplishes the objective of a DC TIG welder of the SCR type that can be operated at a low current without SCR spikes. If the desired weld current is greater than the peak current of the start pulse, the start pulse is not required.
In accordance with the present invention, there is provided a DC TIG welder operated at a low current level, under about 5 amperes in the gap between an electrode and a workpiece. The DC TIG welder has an adjustable output welding current and is driven by a constant current power supply with a first rectifier having an output current controlled by the firing phase of SCRs in an SCR network, a second rectifier settable at a low current below about 5 amperes, and a sensor for sensing the arc voltage. As so far described, the invention is a somewhat standard DC TIG welder, except that the second rectifier of the prior art is not used for welding, but is used to fill in between SCR spikes to stabilize the arc. The arc voltage sensor is used in practicing the present invention. The welder includes a circuit to deactivate the SCRs when the output welding current is equal to or less than the set low current level of the second rectifier stage. The arc is started by first connecting a normal high frequency generator in the welding circuit to pass a high frequency, high voltage current across the gap. Such high frequency generator is used routinely in starting AC and DC TIG welders operated at welding currents substantially above 5 amperes. In accordance with the invention, the arc starting device comprises a circuit for activating this high frequency generator upon starting of the TIG welder and after the arc voltage is sensed to be greater than a given value. This given value in the preferred embodiment is 35 volts DC. As is well known, the arc voltage of a constant current power supply is greater than this given value, there is no arc. The established open circuit voltage of the constant current power supply is being approached. In the invention, a start pulse device, i.e. software, hardware, etc., creates a current pulse simultaneously with a high frequency generator to cause the SCRs of the first rectifier to output a high current pulse having a peak substantially greater than the low current of the low current power supply. The start pulse is obtained by the controller causing a current to rise toward the peak current. A request for a starting pulse results in a command signal to cause operation of the SCRs of the rectifying network in the first stage. The start pulse is tailored to create an arc across the gap and, in practice, has a peak current of 50-70 amperes with a duration of 30-50 milliseconds. The start sequence is initiated when the voltage across the gap is high, indicative of the absence of an arc. The start pulse is created by the high voltage applied across the gap by the power supply after the gap is ionized by the high frequency. Thus, the pulse is created with the high frequency. The pulse during high frequency positively creates an arc. When the welder is operated at minimum current, the phase controlled stage of the power supply is used merely to create the starting pulse. After the sensed arc voltage is decreased below a given value, indicating that a start pulse current is initiated, the high frequency across the gap is terminated. The minimum current welding continues without SCR spikes.
In accordance with an aspect of the invention, deactivation of the high frequency generator has a time delay after an arc voltage across the gap has been decreased upon creation of the arc. The low current or minimum current rectifier stage of the power supply is used exclusively for low current welding at the set current of the second rectifier. This rectifier is adjusted to a weld level in the range of 2-5 amperes.
If and when the welding current demand is for a weld level exceeding the current of the low current second rectifier stage, the phase controlled rectifier stage is initiated to provide the additional current. However, the present invention is directed to a DC TIG welder designed for industrial use and with a rating over 100 amperes that can be operated at a minimum current in the general range of 2-5 amperes without the need for phase controlled current pulses. To accomplish this objective, the non-phase controlled rectified stage of the power supply is adjustable by a variable resistor to obtain the desired open circuit voltage and the desired low current output without the need for current from the SCRs in the power supply. The variable resistor in the low current stage of the power supply sets the output level of the low current stage. In practice, this low current is 2-5 amperes; however, it has been operated at about 1 ampere.
In accordance with another aspect of the present invention, the phase controlled power supply is used when the required welding current is adjusted above the low current level of the fixed stage of the power supply. The term xe2x80x9cfixedxe2x80x9d indicates that it has been adjusted to a low current weld level sustainable without SCR spikes.
In accordance with another aspect of the present invention, there is provided a method of creating an arc in the gap between the electrode and workpiece of a DC TIG welder operated at a low current level, under about 5 amperes. The welder provides a current with an adjustable level in a series circuit including a gap. The welder is driven by a constant current power supply with a first rectifier having an adjustable output current controlled by the firing phase of an SCR network and a second rectifier with a fixed, but settable low current. To start the TIG welder, there is a device for sensing the arc voltage. This inventive method involves activating a high frequency starting generator upon starting of the welder, but after the arc voltage is greater than a given value. Then the first adjustable rectifier outputs a current pulse with a high-peak current. This pulse involves operation of the SCRs. Then, the high frequency voltage is deactivated.
The primary object of the present invention is the provision of a DC TIG welder with an SCR based DC power source for providing high current, wherein the welder can be operated at a low current without use of SCR spikes. The welder has an arc starting system which is positive in operation and novel in concept.
Another object of the present invention is the provision of a high capacity, SCR based DC TIG welder operable below about 5 amperes, which welder does not cause popping or melting of the tungsten electrode.
A further object of the present invention is the provision of an SCR based, DC TIG welder having a starting device so the welder can be operated at low currents without the need for current from an SCR rectifier stage and can be reliably started and sustained.
Another object of the present invention is the provision of a high capacity, SCR based DC TIG welder operable at a low current where the low current is not established by a phase back of the SCRs.
Yet another object of the present invention is the provision of an SCR based DC TIG welder, as defined above, which TIG welder has a positive arc starting capability by using a tailored starting pulse with a high current peak in combination with a high frequency ionization voltage.
Still another object of the present invention is the provision of a DC TIG welder, as defined above, which TIG welder uses a voltage sensor to indicate the existence of an arc and/or current flow to control the starting mode at currents even less than 2 amperes.
These and other objects and advantages will become apparent from the following description taken together with the accompanying drawings.